The present disclosure relates to radar monopulse signal processing. A monopulse signal processing system or arrangement determines the angle between the radar receive beam axis of a radio-frequency (RF) antenna and a line extending to the apparent source of a received RF signal. The received RF signal may originate as a “skin” or surface reflection of electromagnetic energy impinging on a radar “target”, or it may originate from a transmitter of a signal from the target itself, as might be the case with an identification transponder. Thus, both active and passive sources are included in the generic term “source.”
In the past, the term “radio frequencies” was interpreted to mean a limited range of frequencies, such as, for example, the range extending from about 20 KHz to 2 MHz. Those skilled in the art know that “radio” frequencies as now understood extends over the entire frequency spectrum, including those frequencies in the “microwave” and “millimeter-wave” regions, and up to, and including, light-wave frequencies. Many of these frequencies are very important for commercial purposes, as they include the frequencies at which radar systems, global positioning systems, satellite cellular communications and ordinary terrestrial cellphone systems operate.
Many modern tactical radars use phase shifters to electronically specify the spatial position of the antenna beam without requiring mechanical motion of the antenna. These phase shifters can only be set correctly for a specified frequency. If a waveform is transmitted through the antenna which differs in frequency from that used to steer the position of the beam, an error will be introduced into the monopulse measurements. A monopulse waveform may consist of many subpulses or segments, each of which can provide information about the location of the target. Each segment may have different modulation and different frequencies, which introduce monopulse errors. By proper application of correction factors these errors can be mitigated.
FIG. 1A is a highly simplified representation of a monopulse radar system 8. System 8 of FIG. 1A includes a monopulse transmitting and receiving antenna 10 which has a center 11, an antenna plane 10p, an antenna “broadside” axis which lies along a line 12 perpendicular to antenna plane 10p, and a beam axis which can be steered away from broadside to lie along an another direction line 13. In many reflector antenna systems, the lines 12 and 13 always coincide. In many phased array antennas the lines 12 and 13 coincide only for one beam pointing angle and diverge for all others. A signal source 14, which may result from reflection, lies on a line 16 extending from the antenna center 11. In order to assign directions from the location from which the signal source 14 propagates, it is necessary to assign coordinate axes for the antenna 10. In FIG. 1A, coordinate axis Y 12, is the line extending from the center 11 perpendicular to the plane 10p of the array 10. Two axes X and Z that are perpendicular to axis Y and to each other are defined in the antenna plane 10p. These axes are labeled X and Z. By convention, the positive X axis is defined to the right looking outward from the antenna 10. The positive Z direction is defined in the upward direction.
Spatial directions can be measured by direction angles. The direction angle α0 in FIG. 1A is measured from the positive X axis to the commanded beam direction line 13. The direction angle β0 measured from the positive Z axis to the beam direction line 13. The direction angles to the target 14 may be measured as departures from the direction angles α0 and β0 by the angular differences δα and δβ, respectively. That is, if the target direction lies along the line 16, the departures from the beam direction line or axis 13 are labeled or designated δα and δβ. More commonly, angular departures are measured as differences in direction cosines. In the alpha a coordinate, the departure in the α angle cosine is denoted byΔu=cos α−cos α0 The departure in the beta angle cosine is denoted byΔv=cos β−cos β0 As viewed from the center 11 of antenna 10, the target 14 is displaced from the beam axis 13 by δα from the α0 direction and by δβ from the β0 direction. The angular cosine departures Δu and Δv are determined in a monopulse system by monopulse signal processing performed upon the total antenna received signal, in complex envelope form comprised of three complex signals: Δα, Δβ, Σ. Signal Δα is obtained as the difference of the two vertical halves of the antenna received output. Signal Δβ is obtained as the difference of the two horizontal halves of the antenna received output. The Σ signal is the entire received output of the antenna. These three signals are conceptually separated from each other by circuits associated with the antenna, which separating circuits are illustrated as a block 6. The three signals separated by block 6 are coupled to a monopulse signal processing system included in a receiver processing (PROC) portion 20a of radar 8. The processing is performed on the complex envelopes. Given the antenna signals
Σ, the total antenna output;
Δα, the difference of the half-antenna outputs corresponding to the α direction; and
Δβ, the difference of the half-antenna outputs corresponding to the β direction, the α monopulse ratio ρα, and the β monopulse ratio ρβ are formed as follows
            ρ      α        =          Re      (              Δα        Σ            )                  ρ      β        =          Re      (              Δβ        Σ            )      From these quantities, the increments in the direction cosines are obtained. These are, as indicated earlierΔu=cos α−cos α0 Δv=cos β−cos β0 where, as indicated above, α0 and β0 are the commanded “steering” angles, and cos α0 and cos β0 are the corresponding angle cosines. Then Δu and Δv are obtained by insertion of ρα and ρβ into odd degree polynomialsPα(ρα)andPβ(ρβ)These polynomials are obtained in known fashion by antenna calibration.
Also in the arrangement of FIG. 1A, a transmitter Tx illustrated as a portion 20b interacts with the remainder of system 8 to transmit a coded signal including a plurality of subpulses. Transmitter portion 20b may use the antenna 10 for transmission, or it may use some other antenna, as known in the art. In general, the number of subpulses to be transmitted in each pulse can be selected arbitrarily. The transmitted pulse is divided into a number of subpulses. Each subpulse, when properly isolated from the other subpulses and separately filtered, is selected to have the necessary bandwidth and transmitted power to satisfy the system bandwidth required for a specific or selected range resolution and sufficient power to satisfy the desired target detection range. The number of subpulses is limited, in general, by the beam-steering capability in terms of the number of beam positions that can be achieved within the constraints of the available equipment. In a particular embodiment of the disclosure described herein, the number of subpulses per pulse is selected to be four as illustrated in FIG. 1B. More or fewer subpulses per pulse may be appropriate for systems with lesser or greater constraints. The purpose of the use of plural subpulses per pulse is to obtain several or plural values of Δu and Δv, which can be averaged in order to mitigate or ameliorate perturbations occasioned by factors including noise. The transmitter portion 20b of FIG. 1A also includes a beam steering controller BSC that provides the angle steering command to the antenna 10. The beam steering controller and the waveform generator are illustrated as blocks 48 and 52, respectively, in FIG. 2.
For purposes of explanation, four subpulses are assumed. FIG. 1B is an amplitude-time plot 21 illustrating a time-sequential set of four subpulses designated 1, 2, 3, and 4. The subpulses differ from each other in frequency. More particularly, subpulse 1 may be at a frequency of fBAND−A MHz, subpulse 2 may be at a frequency of fBAND−B MHz, subpulse 3 may be at a frequency of fBAND+C MHz, and subpulse 4 may be at a frequency of fBAND+D MHz, where frequencies A, B, C, and D are different offset frequencies, much smaller or less than the electromagnetic carrier frequency used for steering. The frequency fBAND may have a multiplicity of values. A monopulse system may also operate in a passive mode in which the radar antenna acts only in a receive mode. In this passive mode, the radar acts as a passive receiver of transmissions from the target acting as source transponder, where the angle of arrival coordinates are determined by the radar in its receive mode. In such a passive or receive-only radar mode, the arriving electromagnetic wave will have a signal structure as a function of frequency fBand−A MHz, where the value of fBAND is preselected by auxiliary communications with the radiating source. The signals structure in the passive radar mode is illustrated in FIG. 1C.
FIG. 2 is a simplified block diagram illustrating details of receiving and monopulse processing system 20a of FIG. 1A. A signal processor in accordance with this disclosure may operate in either an analog or digital manner, in accordance with its construction. However, a digital processor is preferred.
The monopulse processing system 20a of FIG. 2 includes a receiver and a matched filter system, illustrated as a block 22, for proper filtering of the received signal. The output from the receiver and matched filter system 22 is generated on a set of paths designated together as 24, and may be viewed as including three complex envelope signals, namely Δα, Δβ, and Σ. These three complex envelope signals are the outputs from system 22 and are coupled by paths 24 to a bank or set 32 of three complex analog-to-digital converters (ADCs) 321, 322, 323. In response to timing signals from a controller or radar control computer 90, the bank 32 of complex A-to-D converters simultaneously converts the complex envelopes of each of the three signals Δα, Δβ, and Σ from the matched filter system 22 into three separate complex binary (digital) values. In a typical system, the bank of A-to-D converters 32 may provide each complex envelope component in the form of the components (real and imaginary) of the complex envelope, and any number of bits may be used. Each time the controller 90 activates the bank 32 of A-to-D converters, each of these converters provides a new complex value at its output and on a signal path 28. More particularly, the digitized output from ADC 321 is applied to a path 281, the digitized output from ADC 322 is applied to a path 282, and the digitized output from ADC 323 is applied to a path 283. As a group, these A-to-D converters together provide a new set of these three complex envelope values each time a conversion is commanded. If digital beam forming is employed in a phased array antenna, then the digital beam former provides the same three complex outputs from the ADCs 32. The complex envelope values from ADC 322, representing the “sum” or Σ signal, are provided to a target detection processor 34 for use in determining whether a target is present in the portion of the return signal to which these digital values correspond. The complex digital values of the three outputs of the analog to digital converters ADC 321, 322, 323 are provided to a monopulse signal processing computer designated generally as 40, which is illustrated as including a “prior art” portion 40a joined by a path 41 to a portion 40b according to an aspect of the disclosure, which together provide as outputs values of ΔuTAR=Δ cos α and ΔvTAR=Δ cos β, which are the corrected angle cosines between the line 16 extending to the target 14 and the antenna beam axis 13 of FIG. 1 in the alpha and β directions, respectively. The direction angles α and β may be termed “traverse” and “co-elevation” angles (sometimes known as azimuth and elevation), respectively. The corrected values of u and v, the direction cosines, are applied to tracker 95 as ΔuTAR and ΔvTAR.
Target detection processor 34 of FIG. 2 determines whether the received signal values indicate the presence of a target. If they do, then the detection processor 34 provides to tracker 95 a detected-target signal which specifies the position of the target. If they do not indicate the presence of a target, then either no signal or a no-target signal is provided to tracker 95 by detection processor 34. The monopulse processor provides angular cosine coordinates to target tracker 95, which tracks the locations of the various targets in known manner with the aid of signals from target detection processor 34 and control signals from controller 90.
The monopulse signal processing computer 40 of FIG. 2 ultimately provides to tracker 95 a set of target angle coordinates, in the form of angle cosines ΔuTAR and ΔvTAR, for each set of received input values. These target angle signals specify the angle between the beam axis and the target in the event that the processed values include target energy. If tracker 95 receives a detected-target signal in conjunction with a set of the target angle signals, then tracker 95 determines the target position from the known beam position in combination with the determined range and direction cosines of the target. Tracker 95 then determines whether this target location is a newly detected target or is the new position of an old or previously identified target. If it is a new target, the tracker establishes a new target track to begin following this target. If it is a new position of an old target, then this new position is used to update the track on that old target by providing this new position as the most recent target location. When no detected-target signal or a no-target signal is received in conjunction with a set of target angle cosine signals, tracker 95 discards those angle cosine signals without further processing. Both the target detection processor 34 and the tracker 95 are conventional, aspects of the present disclosure being concerned with the monopulse signal processing 40 which converts the received digital values to their corresponding monopulse ratios and then to target angles cosines.
Significant discrepancies or errors have been found when comparing the target angle as determined by skin reflections with those determined by an active source on the target, such a transponder. Improved or alternative monopulse processing is desired.